Shares in South Korean bio-pharmaceutical firm Celltrion Inc (068270.KQ), which is up for sale, tumbled as much as 14.8 percent on Monday after reports that financial regulators are pursuing its chief executive over suspicious sharemarket trades. Yonhap news agency and Korean broadcaster SBS CNBC said in unsourced reports that regulators had made a preliminary conclusion in the matter regarding Celltrion chief executive Seo Jung-jin at a meeting on Friday.